1. Field of Invention
The present invention relates to a switching mode power converter and more particularly relates to a flyback power converter.
2. Description of Related Art
A flyback power converter typically includes a PWM controller, a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a transformer, and a feedback-control circuit. The purpose of the feedback-control circuit is to sense the output voltage and/or the output current of the secondary-side of the power supply, and to supply a feedback signal to the PWM controller through an isolated device such as an optical-coupler.
FIG. 1 shows a traditional flyback power converter. Although this circuit is able to regulate the output voltage and output current, it has several drawbacks. One drawback of this circuit is its large size, due to the need for an optical-coupler and a secondary feedback-control circuit. Another drawback of this flyback power converter is high power consumption. To maintain a constant secondary-side output current, this circuit includes a current-sense resistor. The current-sense resistor significantly increases the power consumption of the power converter.
In recent years, several primary-side control schemes for flyback power converters have been proposed. These prior-art primary-side control schemes have attempted in various ways to reduce the size and the cost of flyback power converters. One prior-art primary-side control scheme is “Switching Power Supply Packages” by Arthur J. Collmeyer, Mark D. Telefus, Dickson T. Wong, and David B. Manner (U.S. Pat. No. 6,434,021). Although this circuit is able to regulate the output voltage and the output current, it has several drawbacks. One drawback of this prior-art invention is that the feedback control voltage is sensed from a high voltage source. This method results in a loss of accuracy, and it increases the cost of the controller. Another drawback is that the voltage drop of the output rectifier is not compensated for. Generally, the voltage drop of the output rectifier decreases in response to temperature increases, typically at a rate of 2 mV/° C. Thus, the output voltage of this prior-art invention will deviate significantly from a constant DC level.
Another prior-art control scheme is “Method and Apparatus Providing a Multi-Function Terminal for a Power Supply Controller” by Balu Balakrishnan, Alex B. Djenguerian, and Leif O. Lund (U.S. Pat. No. 6,538,908). The drawback of this prior-art is that the optical-coupler and the secondary feedback circuit are still required for loop control. Otherwise, the output voltage and the output current will fluctuate significantly.
Reflected voltage control has also been proposed as a means for primary-side control. Two prior-art patents teaching this method include “Switched Mode Power Supply Responsive to Voltage across Energy Transfer Element” by Balu Balakrishnan, David Michael, and Hugh Matthews (U.S. Pat. No. 6,233,161) and “Switched Mode Power Supply Responsive to Current Derived from Voltage across Energy Transfer Element Input” by Balu Balakrishnan, David Michael, and Hugh Matthews (U.S. Pat. No. 6,480,399).
One principal drawback of these two prior-arts is inaccurate feedback control. In order to generate a feedback control signal, the reflected voltage of the transformer is filtered and turned into a DC voltage (or current) through a resistor-capacitor circuit. However, this reflected voltage signal is not directly proportional to the output voltage, because of the spike voltage generated by the leakage inductance of the transformer. Thus, the output voltage of this prior-art invention will deviate significantly from a constant DC level. Furthermore, the voltage drop of the output rectifier is not compensated for in the feedback loop. When load changes occur, this problem will introduce additional distortion into the output voltage.
Another drawback of these two prior-art inventions is high power consumption. The reflected voltage is filtered to supply power for PWM control. However, the resistor in the filter burns the majority of the reflected power, even if the power consumption required for PWM control is low. Therefore, the power consumption of the power supply is high.
Thus, a need still remains for an efficient primary-side flyback power converter with a well regulated, constant, output voltage and output current.